The present invention relates to improvements applied to fuel injectors for a turbomachine engine. More particularly the invention relates to adjusting the rate at which fuel is injected into a turbomachine combustion chamber.
In conventional manner, a turbomachine engine comprises a plurality of injectors enabling the combustion chamber to be fed with fuel and air during starting and normal operation of the turbomachine engine. There exists two main types of injector: xe2x80x9caeromechanicalxe2x80x9d injectors designed for two fuel flow rates (a primary rate and a secondary rate) depending on the operating stage of the engine (lighting, from low to full power), and xe2x80x9caerodynamicxe2x80x9d injectors which have a single fuel circuit for all operating stages. The present invention relates more particularly to injectors of the first category.
In conventional manner, an aeromechanical fuel injector for a turbomachine engine comprises two fuel feed circuits: a primary circuit corresponding to low feed rates (for use, for example, during a lighting stage and when the engine is operating at low power), and a secondary circuit which comes into action for medium and high rates of flow (for use, for example, at subsequent stages of operation all the way to full power).
That type of injector includes in particular a stop valve which is designed to open at a first predetermined fuel feed pressure and to remain open at higher pressures in order to feed the primary fuel circuit. A metering valve is also arranged to open under a second predetermined feed pressure higher than the first predetermined pressure and to remain open in response to any increase in said pressure, serving to supply the fuel feed flow of the secondary circuit. The secondary flow rate is adjusted by means of metering slots provided in a valve head and providing flow sections that vary as a function of the applied feed pressure: the higher the applied pressure, the larger the flow sections of the slots.
In practice, it is found that in a combustion chamber fed with fuel via a plurality of injectors of the kind described above, when the injectors are all subjected to the same feed pressure corresponding to medium flow rates, they deliver flow rates that are not uniform from one injector to another. This non-uniformity is due mainly to manufacturing dispersions between their respective metering valves, and it can be as great as 10%. The manufacturing or machining tolerances on the metering valves and their metering slots mean that the medium rates of fuel flow cannot be made to be identical for all of the injectors of a given engine. FIG. 3 shows clearly the flow rate differences xcex94D1 and xcex94D2 that can exist in two different injectors 100 and 102 belonging to the same combustion chamber as compared with the design xe2x80x9cidealxe2x80x9d rate 104. At medium rates of flow this gives rise to non-uniformity between injectors which is harmful for proper operations of the turbomachine engine.
The present invention thus seeks to mitigate such drawbacks by proposing a fuel injector which enables non-uniformity between injectors to be decreased. Another object of the invention is to propose an injector that is simpler to make and to improve the adjustment performance at medium rates of fuel flow.
To this end, the invention provides a fuel injector for a turbomachine engine, the injector comprising an injector body having pressurized fuel admission means, a first valve mounted downstream from said pressurized fuel admission means and arranged to open in response to a predetermined fuel pressure defining a first threshold pressure so as to admit fuel into the injector body, a second valve mounted downstream from said first valve and capable of opening in response to a second fuel threshold pressure higher than said first threshold pressure so as to meter at least a fraction of the fuel admitted into said injector body for utilization means for using said fuel, the metered fuel flow rate to the utilization means being a function of flow sections formed through said second valve, the injector further comprising a diaphragm placed between the pressurized fuel admission means and the first valve so as to set the rate at which fuel is admitted into the injector body at a determined value.
The rate at which fuel is admitted into the injector body is a function in particular of the diaphragm. It is fixed at a determined value by selecting a diaphragm opening as a function of technical characteristics that are specific to each injector in the same combustion chamber. More precisely, the diaphragm selected for each injector is chosen as a function of the departure of its flow rate from a design medium flow rate. Thus, each injector in a given combustion chamber is provided with a diaphragm which may differ from one injector to another. As a result, any risk of the various injectors delivering different flow rates is eliminated. In addition, it is easy to replace a diaphragm since it does not require both valves to be disassembled.
Advantageously, the injector further comprises adjustment means for defining a third fuel threshold pressure higher than said second threshold pressure, from which the metered flow of fuel to said utilization means takes place at a rate which is a function solely of the fuel feed pressure.
The adjustment means for defining a third fuel threshold pressure advantageously comprise an abutment for limiting the stroke of the second fuel metering valve.